The present invention relates to a brushless motor control method and a brushless motor controller. More particularly, it relates to the so-called sensorless controller that detects the rotation position of the rotor of a brushless motor without using magnetic detecting means, such as Hall devices.
A conventional technology for the so-called sensorless controller, wherein the rotation position of the rotor of a brushless motor is detected without using magnetic detecting means, such as Hall devices, is disclosed in Japanese Laid-open Patent Application No. Hei 7-123773 in particular.
A conventional brushless motor controller will be described below.
FIG. 38 is a block diagram showing a system configuration of a conventional brushless motor controller. In FIG. 38, a brushless motor 101 is provided with a stator 102 wherein current-flowing phase windings 121u, 121v and 121w are wound on a stator core (not shown), and a rotor 103 having magnets. The phase windings 121u, 121v and 121w are connected to a switching circuit 104, and voltages applied to the phase windings 121u, 121v and 121w are controlled. A voltage is supplied to this switching circuit 104 from a DC voltage source 105.
In the switching circuit 104, two switching devices disposed on the upstream and downstream sides of the current-flowing direction are connected in series, and this series circuit is provided for one phase. Hence, three series circuits are available for U, V and W phases. As shown in FIG. 38, the series circuit for the U phase has an upstream-side switching device 141u and a downstream-side switching device 142u. The series circuit for the V phase has an upstream-side switching device 141v and a downstream-side switching device 142v. The series circuit for the W phase has an upstream-side switching device 141w and a downstream-side switching device 142w. 
In addition, in the switching circuit 104, diodes 143u, 144u, 143v, 144v, 143w and 144w are connected in parallel with the switching devices 141u, 142u, 141v, 142v, 141w and 142w, respectively, in opposite directions.
The phase winding 121u of the brushless motor 101 is connected to the connection point of the U-phase switching devices 141u and 142u of the switching circuit 104. Similarly, the phase winding 121v of the brushless motor 101 is connected to the connection point of the V-phase switching devices 141v and 142v, and the phase winding 121w of the brushless motor 101 is connected to the connection point of the W-phase switching devices 141w and 142w. 
A selector 106 selects one nonenergized phase from among the three-phase windings depending on the switching state of the switching circuit 104, and an A/D converter 107 converts the analog value of the terminal voltage of the selected phase into a digital value. A control section 108 determines the change rate and commutation time of a detected voltage and outputs a driver signal to a driver 109. The switching devices 141u, 142u, 141v, 142v, 141w and 142w of the switching circuit 104 are controlled by the driver 109.
Next, a control method for the conventional brushless motor controller configured as described above will be described below. FIG. 39 shows the terminal voltages and a processed waveform when the brushless motor 101 is driven by the conventional controller in accordance with 120-degree switching, each winding of the stator 102 is energized and controlled at ideal commutation timing, and the brushless motor 101 rotates at a constant speed.
The part (a) of FIG. 39 shows a terminal voltage waveform generating in the U phase. In addition, the part (b) of FIG. 39 shows a terminal voltage waveform generating in the V phase. The part (c) of FIG. 39 shows a terminal voltage waveform generating in the W phase. Furthermore, the part (d) of FIG. 39 shows a processed waveform. The processed waveform is obtained by detecting the terminal voltage of a nonenergized phase in synchronization with a PWM signal at points wherein the detection is possible. At points wherein the detection is impossible, the processed waveform is obtained by carrying out extrapolation on the basis of a change amount with respect to a detection time.
An actual method of obtaining the waveform shown in the part (d) of FIG. 39 will be described below. In the waveforms shown in the parts (a) to (c) of FIG. 39, effective induced voltage information has discrete values, such as the values at points A and B, detected in synchronization with the PWM signal in a period designated by Ts. Hence, for example, induced voltage information in a period Tx shown in FIG. 39 cannot be detected. To solve this problem, the change rate of the induced voltage information with respect to time is obtained at two or more points, such as the points A and B, wherein the detection is possible. This change rate is used to estimate how the induced voltage changes in the period Tx. This kind of extrapolation process is carried out, and the signals for the three phases are connected, thereby obtaining the processed waveform shown in part (d) of FIG. 39.
Next, a method of determining commutation timing in the conventional brushless motor controller will be described below.
In the processed waveform shown in the part (d) of FIG. 39, commutation times are assumed to be flection points, such as times t1 and t2, at which the estimated induced voltages of phases adjacent to each other intersect. These flection points are ideal commutation times for the motor (a state wherein the maximum output can be delivered). If the estimated induced voltages of phases adjacent to each other have a deviation, it is judged that the position of the rotor is not ideal, and the commutation timing is corrected so that the deviation disappears.
As described above, the conventional brushless motor controller drives the brushless motor while estimating the position of the rotor.
However, the controller disclosed in Japanese Laid-open Patent Application No. Hei 7-123773 requires two or more values to obtain the change rate of the induced voltage with respect to time. Hence, the number of detections of the induced voltage changes greatly depending on the rotation speed of the motor. As the speed becomes higher, the number of detections of the induced voltage decreases significantly. Hence, when the speed becomes a value wherein two or more induced voltage values cannot be detected, the change rate of the induced voltage with respect to time cannot be obtained, whereby the motor stops, resulting in a problem.
As a method of solving this kind of problem, a brushless motor driving method is disclosed in Japanese Laid-open Patent Application No. Hei 9-154294.
This conventional technology disclosed in Japanese Laid-open Patent Application No. Hei 9-154294 basically uses the same control method as that of the above-mentioned technology (disclosed in Japanese Laid-open Patent Application No. Hei 7-123773) to drive brushless motors. However, in the conventional technology disclosed in Japanese Laid-open Patent Application No. Hei 9-154294, brushless motor control is carried out by using one induced voltage detection value.
A back emf (electric magnet force) V0 is proportional to the rotation speed N of the motor in accordance with the principle of the motor and is represented by the following equation (1) using a back emf (electric magnet force) constant Ke.V0=Ke×N  (1)
Since the induced voltage is proportional to the back emf, the change amount of the voltage in a unit time is also proportional to the rotation speed N of the motor. The change rate (Δv/Δt) of the induced voltage with respect to time is calculated by the following equation (2).Δv/Δt=α×N  (2)
In the equation (2), a designates a constant inherent in the motor, representing the change rate of the induced voltage with respect to the rotation speed N of the motor. The graph of FIG. 40 shows that the induced voltage is detected during motor driving, and that the change amount of the detected voltage is converted into the change amount of the voltage in a unit time. Therefore, in the case when two or more induced voltage values can be detected, the change amount of the induced voltage with respect to the rotation speed of the motor is calculated. When the number of detections of the induced voltage decreases, the change rate of the induced voltage with respect to the rotation speed of the motor is calculated and used for control. As an actual calculation method, extrapolation is carried out by approximating portions other than two points on the graph of FIG. 40 using a linear function.
In this case, the above-mentioned control method can drive the brushless motor by using extrapolation values obtained from even one induced voltage value that is detected.
As described above, when two or more induced voltage values cannot be detected in principle in the conventional technology disclosed in Japanese Laid-open Patent Application No. Hei 7-123773, the change rate of the induced voltage with respect to the detection time cannot be calculated. Therefore, no commutation time can be determined, whereby the motor stops, resulting in a problem. Furthermore, in the control method of this conventional technology, the time when the estimated induced voltages of phases adjacent to each other intersect is assumed to be commutation timing. However, this control method is applicable only to a surface magnet brushless motor. In the case of an embedded magnet brushless motor, it is known that higher efficiency is obtained by carrying out commutation at a time earlier than the time when the estimated induced voltages of phases adjacent to each other intersect.
When two or more induced voltage values cannot be detected in the conventional technology disclosed in Japanese Laid-open Patent Application No. Hei 9-154294, the change rate of the induced voltage with respect to time is calculated as a value proportional to the rotation speed of the motor in accordance with the above-mentioned equation (2). This control method of Japanese Laid-open Patent Application No. Hei 9-154294 is applicable to only a surface magnet brushless motor, just as in the case of the control method of Japanese Laid-open Patent Application No. Hei 7-123773. In the case when the brushless motor is an embedded magnet brushless motor, the equation (2) is not established. This is because the detected induced voltage of the embedded magnet brushless motor includes a reluctance component due to a motor current.